948 SUMMARY OF CURRENT RESEARCHES RELATING TO 



in fact, and generally smaller also in comparison witli the solid parts 

 of the shell ; but the light is reflected from the surface in the same 

 way, and the experiment ends with a conviction that the differing 

 methods of examination all lead to the same conclusion. 



In examining diatoms as opaque objects with the middle and low 

 powers, the appearances vary more than they do with the vertical 

 illuminator, because, as the light is necessarily oblique, its variations 

 of direction produce changes of appearance. Parts which look dead- 

 white with the vertical light may appear dark, and the thicker por- 

 tions of a shell also change colour ; but the changes of manipulation 

 of the mirror give so many variable experiments as to end in strong 

 confirmation of the results reached by the other means. In opaque 

 mounts the thinness of the shell is shown better than in any other 

 form of preparation. From the dense Eupodiscus argus we find every 

 degree of diminishing thickness till we come to an Actinocyclus lying 

 upon the black slide, its flat disk as black as the background itself, 

 except when the tiny white spots of the areolae pick out the pattern of 

 its marking, or the projecting ring of the valve marks its circumfer- 

 ence. So a Podosira or Cyclotella will be seen, the merest soap- 

 bubble with its play of colours and its manifest tenuity, speaking 

 plainly of the extreme delicacy of the film of silex. 



We will pass over the irregular disk forms for the present, and 

 next consider some of the Naviculce. 



In studying the Naviculce we begin with the large Pinnularia, 

 where the size of the valves and the simplicity of the marking make 

 easy the application of the criteria we have already established. 

 Using transmitted light, the raphe is found to show the colour of the 

 general background, whilst the smooth longitudinal portion of the 

 valve next it is tinted with the pink colour which indicates thickening 

 of the silex. The central nodule shows this in a higher degree, with 

 lenticular effects. The costee are pink in tint also, and in large speci- 

 mens of P. major the interspaces between the ribs are often divided 

 into what appear to be two large oval depressions, of which that next 

 the mid-rib is the shallower, as is shown by its excess of colour over 

 the outer one. The central nodule often extends considerably beyond 

 the inner end of the median line, which is a little enlarged, and seems 

 to terminate in a circular dot, which, by its bright light and freedom 

 from colour, should be a depression reaching nearly or quite through 

 the nodule. In a specimen of P. aljoina from a Scotch gathering I 

 have found a valve turned partly on one side, so as to give an obliquely 

 transverse view through the valve, and in this the enlargement of the 

 median line is plainly seen to extend like a tube through the thick 

 prolongation of the central nodule. It is not uncommon to find 

 broken valves of Pinnularia in which the costae project boldly beyond 

 the interspaces of which the thin film has been partly broken away. I 

 have noticed a specimen of P. divergens in which the thin film has been 

 almost wholly removed by some accidental grinding process and the 

 costfe stand out along each side like the teeth of a comb. Prof. H. L. 

 Smith has given me another similar example of P. major. The raphe 

 appears to be like a channel having a very thin film at the bottom, 



